Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2002-10-08
2004-10-19
Wong, Don (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S096000
Reexamination Certificate
active
06807215
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to tunable semiconductor lasers and particularly to single mode lasers for use in wavelength division multiplexed systems.
BACKGROUND OF THE INVENTION
Single mode wavelength tunable semiconductor lasers are important components in wavelength division multiplexed (WDM) systems. In such applications, the essential parameter that determines the effectiveness of a tunable laser is the inter-channel switching time and the simplicity of the emission-wavelength controlling circuit. Currently, many fixed wavelength laser sources, such as distributed feedback (DFB) lasers, rely on temperature tuning to switch the wavelength between the various channels. In this context, the temperature of the laser is changed to create variations in the bandgap energy and the refractive index, leading to a change in the emission wavelength. The process is relatively simple, but is inherently too slow for fast switching applications.
An alternative approach to wavelength control is through electronic tuning, a technique which can be applied to a two-section distributed Bragg reflector (DBR) laser having a gain section and a Bragg section for tuning. The tuning of the output wavelength can be achieved by varying the wavelength &lgr;
B
of the Bragg section which satisfies the Bragg condition for constructive interference:
&lgr;
B
=2n
eff
&Lgr;
g
in which n
eff
and &Lgr;
g
are the effective refractive index and the grating period of the Bragg section, respectively. In this approach, electrical current is injected into the Bragg section of the laser such that it is perpendicular to the surface of the device. This injection of current causes a change in the carrier density and a corresponding change in the effective refractive index, thereby altering the lasing wavelength that satisfies the Bragg condition. This technique of electronic wavelength tuning allows faster channel switching, such that the switching time is limited only by the speed of the integrated driving and controlling circuitry.
However, two-section DBR lasers suffer from discontinuous wavelength tuning, due to mode jumping from one longitudinal mode of the laser cavity to the next. Typically, a jump occurs every 2-3 nm over a 15 nm tuning range. This so-called mode-hopping occurs primarily because of interference and competition between the optical mode that is determined by the Bragg section and other residual longitudinal Fabry-Perot modes arising within the laser device. For a simple two section device comprising gain and Bragg sections, longitudinal Fabry-Perot modes can arise from cavities formed by Fresnel reflections at the air-gain interface, gain-Bragg region interface and air-Bragg region interface.
In order to reduce optical loss in the Bragg section of the DBR laser, so that higher output power can be achieved, it is desirable that the bandgap energy of the Bragg section be larger than that of the gain section. This is usually accomplished by a growth and regrowth technique resulting in different material composition in the waveguide layers of the gain and Bragg sections. However, this introduces a large mismatch in the transverse refractive index profile of the two sections, leading to the unwanted reflection at the gain-Bragg section interface. For example, a 100 nm detuning in the band edge of the Bragg section relative to the gain section, for a device based on an epitaxial layer of InGaAsP, would result in an abrupt refractive index difference of 0.08 leading to a gain-Bragg interface reflectivity of approximately 2×10
−4
, if the waveguides in the two sections are perfectly aligned. However, such perfect alignment is almost impossible to realize given the typical nonuniformity across an epitaxially grown wafer. Thus, if there is also a vertical offset between the waveguides in the two sections, the interface reflectivity increases to 5×10
−4
. Variation of the waveguide thickness across the interface will lead to an even higher value.
Even if the exposed facets of the laser device are coated with anti-reflection optical coatings, it is extremely difficult to completely eliminate all the reflections. In fact if, as is usually the case, the laser output is taken from the DBR end of the device, the facet at the opposing end is coated with a high reflectivity (HR) coating at the lasing wavelength, which can further exacerbate the problem of parasitic modes. The situation is even more severe for three, four or five section tunable laser diodes.
However, despite the increased number of Fresnel reflections, the conventional approach to solving this problem is to use a three-section DBR laser, comprising gain, phase and Bragg sections as described by S. Murata, I. Mito and K. Kobayashi in “Tuning ranges for 1.5 &mgr;m wavelength tunable DBR lasers”, Electron. Lett., vol. 24, pp. 577-579, 1988. In the three section laser, electrical current is also injected into the phase section to adjust the phase of the feedback from the DBR by means of carrier-induced refractive index changes. With the addition of a wavelength reference, this permits stabilization and fine-tuning of the mode frequency, resulting in continuous tuning of the device output wavelength over the entire available range. Other structures derived from the DBR laser are also used, including: sampled grating DBR (SG-DBR) described in C. K. Gardiner, R. G. S. Plumb, P. J. Williams and T. J. Ried, “Wavelength tuning in three section sampled grating DBR lasers”, Electron. Lett., vol. 31, pp. 1258-1260, 1995; superstructure grating DBR (SSG-DBR) described in Y. Tohmori, Y. Yoshikuni, T. Tamamura, H. Ishii, Y. Kondo and M. Yamamoto, “Broad-range wavelength tuning in DBR lasers with super structure grating (SSG), IEEE Photon. Technol. Lett., vol. 5, pp. 126-129, 1993; and grating coupled sampled reflectors (GCSR) described in M. Oberg, S. Nilsson, K. Streubel, J. Wallin, L. Backbom and T. Klinga, “74 nm wavelength tuning range of an InGaAsP/InP vertical grating assisted codirectional coupler laser with rear sampled grating reflector”, IEEE Photon. Technol. Lett., vol. 5, pp. 735-738, 1993. All these devices are capable of continuous tuning with a larger wavelength tuning range.
However, these types of laser suffer from increased complexity of driving circuitry as compared to the two-section DBR laser. More electrodes (at least three) are required and matching of numerous input currents to the appropriate electrodes is necessary to accurately select a wavelength. In addition, as the device ages under conditions of constant electrical current bias, the carrier density in the semiconducting material will tend to decrease, owing to nucleation of non-radiative defects or increased leakage around the active layer. This leads to a drift of the emission wavelength which, in time, can become significant and may even lead to mode hopping. The use of multiple electrodes in tunable lasers such as the SSG-DBR requires more complex locking algorithms, with associated look-up tables, and the whole issue of wavelength stabilization becomes more complicated. Calibration of the emission wavelength for multiple-electrode tunable laser diodes is also extremely time consuming. Furthermore, the output power that can obtained from SG-DBR, SSG-DBR and GCSR lasers is comparably small, typically limited to a maximum of 10 mW.
Recently, continuous wavelength tuning has been realized by using a two-section DBR laser, disclosed in H. Debrégeas-Sillard, A. Vuong, F. Delorme, J. David, V. Allard, A. Bodéré, O. LeGouezigou, F. Gaborit, J. Rofte, M. Goix, V. Voiriot and J. Jacquet, “DBR module with 20-mW constant coupled output power over 16 nm (40×50GHz spaced channels)”, IEEE Photon. Technol. Lett., vol. 13, pp. 4-6, 2001. The operation of the laser in a continuous wavelength tuning mode is achieved through a combination of adjusting the gain current injection, Bragg voltage and temperature of the laser. Although the process is relatively simple, mode hopping occurs when the device is operated at a fixed temperature
Chan Yuen Chuen
Choo Lay Cheng
Lam Yee Loy
Lim Hwi Siong
Ong Teik Kooi
Buckley Maschoff & Talwalkar LLC
Denselight Semiconductor PTE LTD
Vy Hung T
Wong Don
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